Radiography is the use of ionizing radiation (such as x-rays) to create internal images of an object or body. By using the physical properties of the irradiating particles, an image can be developed of the target that displays areas of various densities and compositions. Applications of radiography include medical radiography and industrial radiography.
Industrial radiography is a technique used to inspect materials for hidden flaws by using the ability of energetic x-rays and gamma rays to penetrate various materials. A typical configuration for a radiographic device includes a radiation source for emitting the radiation (e.g., x-rays) used for imaging and one or more radiation detectors corresponding to the radiation source for collecting incoming radiation after passing through the target volume. The particles collected by the detectors are subsequently used to generate a display (i.e., one or more images) of the targeted volume.
Generally, the detectors used for x-rays are usually of the scale of the size of the object being imaged. These detectors often comprise electronic circuits in the form of amorphous-silicon (a-Si) thin film transistor (TFT)/photodiode arrays (converters) coupled to radiation scintillators. Scintillators are used as detectors of radiation due to their inherent capability of converting incident radiation into lower-energy photons, e.g., visible light.
A natural extension of industrial radiography techniques for generating discrete images is using the same configuration in the generation of multiple images rapidly in a sequence, which when combined chronologically, can be viewed as a video. Conventionally, flat panel x-ray detectors are popular in industrial radiography applications due to their lower space requirements and generally adequate capabilities. However, flat panel detectors are often limited in frame rate and maximum area. Moreover, due to electrical connections along borders of the rectangular active area, they cannot be configured together to tesselate a plane without gaps between rectangular areas for larger fields of view (e.g., one or more square meters). For these reasons, flat panel detectors are unsuited for high speed radiography of large fields of view.
Another possible solution replaces flat panel x-ray detectors in favor of discrete-channel x-ray detectors. Discrete channel x-ray detectors are commercially available that operate at very high frame rates but are impractically expensive for large areas. Discrete channel detectors can be extremely fast, and can have very good x-ray detection efficiency, but with electronics required for each pixel the cost becomes prohibitively high for square meters of coverage
Yet another x-ray imaging system uses vacuum tube image intensifiers to improve light yield from x-ray input to optical output, but like discrete-channel x-ray detectors, they are not practical for larger fields of view. Single video camera systems are limited in the number of pixels per frame, and therefore the detector is essentially limited by the number of pixels in the camera system. One camera can image a large area detector, but the pixels will be larger, dividing the large area into the same number of pixels in the camera sensor. Commercially available image intensifiers are inherently fast enough for high speed video applications, but are inefficient detectors at megavolt energies and cannot be made large enough to cover larger areas either.